Don't stop/restart a stream that already has the correct output device.

This prevents an audible audio glitch when setting the audio output
device ID for a tab with multiple open streams. Before this change,
there would be an audible gap in all streams. After this change, there
is no audible glitch for streams already playing via the correct
device.

This is also a minimal change to enable a JavaScript-based workaround
to issue 317900, the workaround being that you can call setActiveSink
repeatedly without any negative consequence.

TBR=tommi@chromium.org
BUG=298158,317900

Review URL: https://codereview.chromium.org/68523013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235170 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235170